I really like gaggedLouise's answer, so let mine be just a small supplement to that.
Firstly, understand that gold is almost forever. It does not tarnish, reacts with every few things, and is extracted much more easily by platinum, this allowing previous civilizations to use it.
Honestly? Gold's worth is, actually, partially because "we say it is", but, in that, it lends itself well to being said. To be more clear, gold's terribly limited quantity allows people to more easily measure value against it, because we can understand that, if we have gold, it will always have value. Gold's appearance allows it to be easily identified compared to the other rare earth metals that otherwise satisfy the "limited quantity" criteria.
If interested, search up "fiat currency", the criteria of which gold partially can satisfy which thus allows it to act as a universal currency.
Although "we say it is" may seem to be a weak reason, think about the fact that gold's value developed separately on each continent. Even before the Europeans had heard of the City of Gold or of how Aztecs valued gold so much, gold was at the top of their list of things to find and have.
So basically, gold satisfies some universal criteria of being a good currency. The reason we do not use it anymore is because it lacks some other criteria, namely, the ability to be controlled. A truly good currency can be adjusted by the government which issues it, and while that was possible in the past, it is not possible anymore.